Various embodiments relate to molecular sieve blends including a blend of a hydrophilic zeolite and a hydrophobic silica binder and processes of use of these molecular sieve blends, such as for dehydration of liquid and gaseous hydrocarbon streams, drying of cracked C1-C4 hydrocarbon gas streams, dehydration of ethanol feed streams, separation of hydrocarbon feed streams, and removal of various undesired materials from various types of feed streams.
Zeolites are hydrated metal alumino silicates having the general formulaM2/nO:Al2O3:xSiO2:yH2Owhere M usually represents a metal of an alkali or alkaline earth group, n is the valence of the metal M, x varies from 2 to infinity, depending on the zeolite structural type and y designates the hydrated status of the zeolite. Most zeolites are three-dimensional crystals with a crystal size in the range of 0.1 to 30 μm. Heating these zeolites to high temperatures results in the loss of the water of hydration, leaving a crystalline structure with channels of molecular dimensions, offering a high surface area for the adsorption of inorganic or organic molecules. Adsorption of these molecules is dependent upon the size of the zeolite channels. The rate of adsorption is limited by the laws of diffusion.
Zeolites are used for a number of processes. The choice of zeolite is important in a number of chemical processes well known to those skilled in the art. For example, catalytic processes of interest using zeolites in the petrochemical industry include reforming, hydrocracking, dewaxing, isomerization, fluid catalitic cracking (FCC), partial oxidation, alkylation and disproportionation of aromatics. Zeolites are also used for dehydration, adsorption of various compounds from feed streams and separation of various hydrocarbons in a feed stream.
Molecular sieves have been advantageous for a number of processes as the diffusion of materials into and out of the pores can be facilitated based on the pore size that is present within the particular molecular sieve. (For purposes of this disclosure “zeolite” and “molecular sieve” have the same meaning.)
One limitation on the utilization of zeolites is their extremely fine particle size. Large, naturally-formed agglomerates of zeolite crystals break apart easily. In addition, because the pressure drop through a bed containing only such fine zeolite crystals is prohibitively high, these zeolite crystals cannot be used alone in fixed beds for various dynamic applications, such as drying of natural gas, drying of air, separation of impurities from a gas stream, separation of some gaseous and liquid product streams and the like. Therefore, it is necessary to agglomerate these zeolite crystals with binder materials to provide an agglomerate mass containing the zeolite crystals, which exhibits a reduced pressure drop.
To permit the utilization of zeolite crystals, different types of clays have conventionally been used as the binder materials for those crystals, wherein the clay binders have generally been selected from attapulgite, palygorskite, kaolin, sepiolite, bentonite, montmorillonite and mixtures thereof, particularly attapulgite.
In one example of the utilization of a molecular sieve adsorbent, water is removed from a cracked gas stream, for example, for the production of ethylene. The molecular sieve adsorbent is utilized immediately before a cryogenic process to remove water so that ice is not created during the process. However, inherent in the process is the fact that the hydrocarbon feed stream contains unsaturated hydrocarbons, such as alkenes, which are very reactive. These unsaturated hydrocarbons tend to form oligomers and polymers, which act as bed fouling agents and are commonly referred to as green oil or coke. These agents block adsorption channels and reduce the working capacity of the bed for dehydration. Accordingly, it is also important that the molecular sieve adsorbents produce very low quantities of green oil or coke during an adsorption process. Many of the clay binders that are traditionally used as binder materials with the zeolites contain metallic acid sites that encourage polymer/oligomer formation by a catalytic reaction. Conventionally, these clay binder materials are treated with a phosphate solution to reduce this catalytic activity. Notwithstanding, there are still issues associated with the production of green oil/coke during processes for treatment of hydrocarbon feed streams when clay materials are used as the binder material with zeolites.
Silica has sometimes been used as a binder material with high silica molecular sieves to form catalyst agglomerates for specialty catalytic reactions, wherein the molecular sieves used have included, for example, ZSM-5, Y zeolites and SAPO zeolites. Because of the hydrophobic nature of both the silica binders and the high silica zeolites, these catalytic materials have been limited in use to organic reactions. For example, these hydrophobic silica binders blended with hydrophobic high silica zeolites have been utilized as catalytic materials in the petrochemical industry for reactions including reforming, hydrocracking, dewaxing, isomerization, partial oxidation, alkylation, disproportionation of aromatics, and particularly as fluid catalytic cracking catalysts. These catalytic reactions conventionally utilize hydrophobic zeolites having a high silica content, wherein the SiO2:Al2O3 ratio is at least 50, preferably greater than 200 and as high as 600 or so. To enhance the high silica content of these zeolites, they are often dealuminized to increase their silica:alumina ratio, making them even more hydrophobic. The silica binders used with these catalysts are also required to be highly hydrophobic. Binders used to produce catalysts for these catalytic reactions are not included within this disclosure. Further, the binders of this disclosure are not conventionally utilized to form these catalysts.
One problem with many conventionally formed zeolite agglomerate blends is decreased diffusion. The larger the diameter of the formed zeolites, the slower the rate of diffusion of the molecules to be adsorbed. Particularly in the field of pressure swing adsorption, this effect is highly adverse to short cycle time and thus to productivity. Enhanced kinetic values or faster mass transfer rates can result in shorter cycle time and lower power consumption and thus higher adsorbent productivity.
It has been recognized that a reduction in the particle size of formed zeolites leads to shorter mass transfer zones and shorter cycle times. This is based on the assumption that the time needed for adsorbates to travel through the macropores of the adsorbents limits the cycle time, i.e. macropore diffusion is the rate limiting step in these processes. The size of the pores in these zeolites can be improved by adding pore forming compounds to the zeolite before the agglomerate forming step.
Accordingly, it is one intent to disclose a process for the production of a molecular sieve blend which is effective and highly selective for the removal of water from hydrocarbon feed streams, such as those containing ethanol or cracked gases.
It is a still further intent to disclose molecular sieve blends which maintain their physical properties and diffusion capabilities even with a reduced quantity of binder than is conventionally used.
It is a still further intent to disclose molecular sieve blends which limit the production of undesired oligomers and polymers during utilization.
It is an additional intent to disclose a process for the preparation of molecular sieve blends with enhanced diffusion rates.
It is a still further intent to disclose a process for the production of molecular sieve blends containing a hydrophobic silica binder that are effective and selective for adsorption processes.
It is a still further intent to disclose a molecular sieve blend comprising a low silica, hydrophilic zeolite blended with a hydrophobic silica binder.
It is a still further intent to disclose a process for drying a feed stream comprising passing the feed stream over a molecular sieve adsorbent blend comprising a low silica, hydrophilic zeolite and a hydrophobic silica binder.
It is a still further intent to disclose a process for the separation of polar materials using a molecular sieve blend comprising a zeolite, particularly a low silica hydrophilic zeolite, more particularly a low silica hydrophilic zeolite 3A, and a hydrophobic binder, particularly a hydrophobic colloidal silica binder, more particularly a hydrophobic colloidal silica binder.
It is still further intent to disclose a process for separation of components of a gaseous or liquid feed stream comprising passing that gaseous or liquid feed stream over a molecular sieve blend comprising a low silica hydrophilic 3A zeolite powder and a hydrophobic colloidal silica binder.
These and other intents are obtained from the processes for production, the processes for use and the products of the various embodiments disclosed herein.